"My name is Sarah Jane Smith and once, I travelled amongst the stars. I saw worlds beyond imagination, I went to places beyond my wildest dreams and met creatures beyond my darkest nightmares...and all of it was amazing. Then one day when I wasn't expecting it, I came home. Life changed, more than I could ever have believed possible. Now, I have a son, even a family of sorts. Life is good on Bannerman Road, even if you sometimes have to work at a weekend..."

Waving goodbye to her son Luke and his friends Clive Langer and Rani Chandra, Sarah Jane drives away from Bannerman Road and, some time later, arrives at Genetec Systems, a gleaming new research facility where its owner, Madison Yorke, tells her about his latest experiments in nanotechnology - the development of microscopic robotic particles capable of completing intricate or dangerous tasks a human cannot perform.

Sarah voices the concerns certain environmental scientists have made about the destructive capability of such robots, but Mr. Yorke is dismissive of the idea, also shrugging off further claims that he has been doctoring his findings to secure financial backing.

Eventually Sarah's questioning gets her thrown out of the building and she is forced to return home.

"As I said, my life changed, but I love my job and my life, because despite all of the amazing things I saw on other planets, I've learned that living on Earth can be just as exciting. If you keep your eyes and mind open you'll find extraordinary things happen everywhere, even on your street; because once you've seen the universe for real nothing ever looks quite the same again."

Just as Sarah arrives home she and her friends look up to the skies to see a massive ball of fire hurtling towards Earth. Clyde excitedly asks if it was something more than a falling meteor, and to find out he, Sarah, Luke and Rani all venture up to Sarah Jane's attic.

Sarah calls on Mr. Smith, her super-intelligent alien computer, to analyse what the object was and he confirms that it was indeed a spaceship, belonging to a Judoon - a race of rhinoceros-like creatures who act as intergalactic police. Although military organisation UNIT have already reached the crash site, he claims that a pod was ejected from the ship before impact and has not yet been recovered.

The four friends hurry outside to Sarah's car but are interrupted by Gita, Rani's mother, who has arrived to ask Sarah if any of her journalistic contacts could prove useful in promoting her floristry business. Sarah is sceptical but gives her the address of Genetec Systems before speeding away with the others.

They arrive on an abandoned estate ten miles from the main crash site, but before they can investigate they are shot at by a Judoon. They quickly take cover and soon realise that it is not they the Judoon is aiming for, it is another creature on the run through the estate - the ship must have been a prison carrier, and its convict has escaped. The Judoon soon disappears in pursuit and Sarah worries about what might happen if the conflict on Earth gets out of hand - Judoon care very little for humans so anyone getting in the way could be dangerous. She vows to help catch the prisoner before anyone gets hurt.

Elsewhere on the estate the Judoon cautiously follows its prisoner but is caught off guard and knocked out by the escapee - a lizard-like creature with a long hissing tongue. It hears Sarah and her friends approaching and flees, leaving them to help the injured Judoon back to its feet. The Judoon, named Tybo, examines Sarah and assimilates her language so as to make communication easier. Immediately he accuses the group of impeding his inquiry and threatens to execute them, but is in no fit state to do so. Sarah asks about the prisoner he has lost and Tybo tells her that he is known as Androvax, the Destroyer of Worlds.

Suddenly a scream is heard and Sarah and Clyde hurry off to find the cause. They arrive in an empty church hall, where they find a small girl stood alone with her back to them. Hearing them she turns around, claiming that 'the monster' has taken her mother. Sarah calls Luke and tells him to keep Tybo busy whilst she looks for Androvax - she needs to find him first to avoid bloodshed.

Luke bluffs to Tybo about Androvax having been sighted heading back to the crash site and the irritable Judoon Captain quickly turns walks off to intercept him, Luke and Rani following close behind.

Back in the church hall, Clyde searches the building for Androvax but the search is fruitless, he ponders why a Destroyer of Worlds would bother to take a little girl's mum, and Sarah worries that she may be a hostage - one Tybo will have little regard for if it comes to a showdown.

The little girl - Julie - recalls seeing Androvax, and is still scared by the encounter. Sarah and Clyde try to reassure her and she seems to calm down, but by now Sarah has had a chance to run a scan of the girl with her wristwatch computer, and confirms it reads her as not being human. Immediately Androvax - the lizard creature - emerges from the girl's body and with a single touch paralyses Sarah. Clyde asks what it is Androvax is after and he claims that he wishes to retain his freedom. He then merges with Sarah's body and, using it as a disguise, sends Clyde into a similar state of paralysis.

Meanwhile, Tybo soon realises that he has been lied to and Luke and Rani try to fob the incident off as a mistake. Citing the phone call as the cause of the 'mistake', Tybo takes both of their mobile phones and stamps on them to prevent further delay.

Still possessed by Androvax, Sarah returns to Bannerman Road whilst Tybo makes his way to the church hall on the abandoned estate. They find Clyde's frozen body and manage to revive him, after which he reveals what has happened to Sarah. As Julie and her mother - also hidden in the hall - start to wake up, the three young humans and their Judoon ally leave. Outside, Tybo intercepts a Police car and commandeers it from the two officers sat inside. He then takes the wheel himself and after promptly snapping the handbrake off, slowly pulls away.

Back on Bannerman Road, the mind-controlled Sarah enters the attic and calls upon Mr. Smith for assistance. Despite his suspicions about Sarah's changing speech pattern and body language, Mr. Smith is forced to comply as Sarah enquires about his ability to infiltrate any computer system on Earth.

On the road, Tybo pulls up at a junction and takes the opportunity to marshal another motorist about the volume of their stereo. After threatening them with his gun they quickly comply and he drives off, still accompanied by Luke, Rani and Clyde.

At Genetec Systems, Gita and her uncooperative husband Haresh pull up outside the building to canvas for business. Haresh worries that their activities might be considered sinister, given that they are turning up unannounced. Undeterred by his pessimism, Gita goes inside and her plant-laden spouse follows.

Tybo and the others arrive back at Bannerman Road but discover Sarah has already left. The Judoon Captain is eager to leave given the lack of a lead in his enquiry, but his human accomplices protest, claiming Sarah may have left a clue as to her new whereabouts. At the moment Tybo receives a signal telling him a full Judoon taskforce is on its way to assist - they have only forty-five minutes before the planet is overrun.

Meanwhile in the laboratory building, Gita and Haresh are discovered by a security guard, who escorts them from reception to another part of the building just as Sarah arrives. She makes her way to one of the main labs where she finds Mr. Yorke, whom she frightens by destroying one of his computers with her Sonic Lipstick. He tries to call security but Sarah claims he has already disabled the building's communication system. She then explains that she has come to take the nanoforms, requiring them to build her something.

Again Mr. Smith tells the children to run but they stand their ground. Tybo threatens to use his gun to destroy the computer before it can detonate, but instead Luke takes centre stage, reasoning with Mr. Smith that since his central protocol is to defend the Earth, and since Sarah's order contradicts this, he must therefore override the command to detonate. After considering this Mr. Smith accepts and cancels the countdown with one second left to spare.

At Genetec Systems Sarah (alias Androvax in disguise) manages to upgrade the nanoforms to widen their capabilities and forces Mr. Yorke to input their activation code. As he does so she reveals her plan for the microscopic droids - they are going to build her a spaceship.

Mr. Smith reveals what Sarah was looking for when she came to see him - she has gained access to the blueprints of the spaceship that crash-landed on Earth in 1947 in Roswell. Luke realises that Androvax plans to rebuild the ship and escape the planet before the Judoon fleet arrives - after being awakened from their sub-zero state of storage; the nanos will be able to construct the ship in less than thirty minutes.

At the lab Mr. Yorke realises that Sarah has reprogrammed the nanoforms to multiply at an accelerated pace, causing them to spill out of the containers and into the open air. He warns that they will devour the planet if not contained but this is all part of Androvax's plan - once the spaceship is constructed he will be able to cover his tracks by destroying Earth.

Meanwhile, Gita and Haresh are still being held by the security guard in his office. When a security alert goes up he is forced to leave them but locks them in to prevent them from causing any more trouble. Once he is gone Haresh begins trying to pick the lock, but with little success. Gita manages to save them time by locating a spare set of keys in the desk drawer.

Luke, Rani, Clyde and Tybo arrive outside the laboratory in time to find the nanoforms swarming on the roof of the building, beginning construction of the ship. Their arrival is detected by Sarah in the main lab, and despite her reflection - which shows the true Sarah hidden beneath Androvax's possession - warning her that they will stop the evil plan; she vows that they will never gain entry to the locked-down building.

Back outside, after Tybo forces them to adhere to the pay and display restrictions in the car park, Clyde, Rani and Luke try to get past the main door. Finding it locked Tybo draws his gun, but before he can fire Mr. Yorke manages to reverse the lockdown and allow them access. As punishment Androvax turns on Yorke and sends him into a paralytic trance.

Downstairs Tybo manages to scare off the security guards in time for his human aides to study a map of the building and head towards the laboratories. On the way Rani spots her parents walking around the facility and does her best to hide herself, her friends and their Judoon guardian.

Gita and Haresh, having gotten themselves lost, are looking for an exit when a squadron of four Judoon officers teleport into the building beside them. As Tybo did with Sarah, one of the troop scans the humans and adopts their language before leading the others away to carry out their tasks, warning that obstruction will result in death.

In the lab Androvax again quarrels with his reflection, belittling the suppressed Sarah for attempting to act as conscience. He threatens to harm her friends if she is not silent, but Sarah remains confident that he will not succeed.

Despite Tybo protesting that they are not authorised to enter the labs, Clyde, Luke and Rani make their way into the test rooms to search for Sarah. The room they try is empty but they do manage to trap Tybo - who has received communication from the recently-arrived troop - in a electro-dampening cupboard to stop him from slowing down their investigation any further and so that the other Judoon will have to find him first before they can look for Androvax, as his radio will not work in the dampening field.

Gita and Haresh watch as the Judoon search the building. It then dawns on Gita that the security guard took her mobile phone when they were caught, and drags her husband away to find it for fear of it, and all of her contacts, being lost.

Luke, Rani and Clyde manage to find Sarah and warn the creature possessing her that the Judoon have started to arrive in the building so he had better leave. Luke tells him to release his mother before he goes, but Androvax claims that he cannot, for he knows that once released, Sarah will not allow him to escape.

The children urge Sarah to fight the being controlling her, claiming she is strong enough to expel Androvax. For a moment it appears to work and Luke embraces her, but this is simply a trap to get him close enough to send him into a trance. This done, she turns on Rani and Clyde and, using the Sonic Lipstick as a remote control, summons a swarm of nanoforms to devour them. They run out of the lab and through the building, the black swarm spreading after them. Narrowly avoiding the Judoon squadron, they lock themselves in another laboratory.

In the main lab Sarah brings Luke out of his trance and takes him to the completed ship. Once inside Luke marvels at the scale of the nanoforms' construction.

Clyde and Rani search for a weapon to fight the approaching swarm and seem to find an answer when they recall Luke's claim that the nanoforms fall dormant in sub-zero temperatures. As the Judoon patrol manages to locate Captain Tybo and release him from the dampening chamber, they grab two fire extinguishers and freeze the swarm and hurry outside to find a way of stopping the robots for good.

Inside the spaceship, Luke urges Androvax to let Sarah go, but he refuses. Through his conversation it becomes evident that Sarah is fighting for control, her voice breaking through occasionally to reveal that Androvax's penchant for destroying other people's worlds was born from the destruction of his own. He admits that his planet was frozen by a dying sun, leaving his people entombed forever and him alone in the universe. His jealousy drove him to show other races the single truth of the universe: destruction.

Clyde and Rani continue their flight but find themselves in the company of Tybo and the other Judoon. Found guilty of multiple offences, they are sentenced to death but before Tybo can fire at them, the nanoforms turn on the building and begin devouring it, causing a tremor that acts as enough of a distraction to allow the two humans to escape. The Judoon follow.

They ascend to the spaceship - having been allowed in by Luke, who recalls all of the controls from the blueprints displayed by Mr. Smith, and watch as the young genius reveals that he has removed a vital fusion mechanism from the ship, rendering it useless. He warns that Androvax can only have it back if he releases Sarah and disables the nanoforms. Having seen the Judoon approaching, the volatile alien is forced to obey. He steps out of Sarah's body just as the Judoon arrive.

The tremors grow fiercer as Androvax reiterates to Luke that the only truth of the universe is destruction. He agrees that demolition is key to evolution, but claims that really the truth of the universe, it's once constant, is survival. He turns away and sets to work at the computer, using it to hack into the Genetec computer system and access the nanoforms' programming. He succeeds, and the swarm surrounding the building are deactivated.

Defeated, Androvax turns on Sarah and warns that, even if not by his own hand, one day she will be destroyed. He is led away by the four Judoon guards whilst Tybo turns to Clyde, Luke and Rani and claims that, having helped secure his prisoner they no longer face the death penalty, rather they are to be grounded henceforth to Earth. He then claims that the Judoon will confiscate the newly-built ship and orders the others to leave.

The four friends make their way outside, narrowly avoiding being seen by Gita and Haresh, and speed away in Sarah's car just as Mr. Yorke awakens and steps out side in time to see the ship take off and blast into the heavens.

Returning home, Sarah claims that although things may die all the time in the universe, just as Androvax claimed, that death is always the start of something new, and that is the stunning thing about the cosmos.

Soon they are joined by Gita and Haresh, who claim that they have seen aliens. Gita urges Sarah to turn their story into an article, and hurries away to prepare to reveal all, whilst Clyde joshes his headmaster about believing in the otherworldly. Mr. Chandra claims that nobody will believe the tale, claiming that to the rest of the world, Bannerman Road is an ordinary place - what could ever happen there? Sarah tells Luke: